The present invention relates to a brake system for an automobile. More specifically, the present invention relates to a master cylinder apparatus used in a brake fluid pressure controlling system for electrically controlling a fluid pressure supplied to wheel cylinders; i.e., a so-called brake-by-wire (BBW) system.
A master cylinder apparatus for a BBW system comprises a master cylinder adapted to be connected to wheel cylinders through fail-safe valves and a stroke simulator for ensuring a desired stroke of a brake pedal upon receiving a brake fluid introduced from the master cylinder. In the event of failure of the BBW system, the fail-safe valves are opened, and the fluid pressure generated in the master cylinder is supplied to the wheel cylinders.
Generally, in a BBW system, a fluid pressure supplied from the master cylinder to the wheel cylinders is controlled on the basis of an amount of movement of a primary piston provided in the master cylinder, i.e., a piston stroke. To detect a piston stroke, as is described in U.S. Pat. No. 6,192,685 B1, use is made of a sensor which detects operation of a brake pedal. In this technique, however, accuracy of detection of a piston stroke is low, due to the effect of mechanical play in a mechanism for providing an operative connection between the brake pedal and the primary piston. As a result, controllability of the BBW system is reduced.
This problem can be avoided by providing a stroke sensor in the master cylinder so as to enable a stroke of the primary piston to be directly detected by the sensor. However, when a conventional linear sensor is used as a stroke sensor, an axial dimension of a master cylinder inevitably becomes large, and therefore a master cylinder apparatus has poor mountability to a vehicle.